Ok, I haven't been playing a long time, but it seems to me that Tristana is best suited to staying OUT of the heat of the battle (with the exception of jumping in for a quick kill with Rocket Jump)

This build focuses mostly on ASPD, Crits, and abusing that ASPD! (And of course her insane range)

Ok, odd picks, right?

Sadly, Tristana does suffer from Mana Lack, and the MP5 helps some there, the added CDR, allows you to keep that ASPD buff up better, and sadly ASPD in runes is pitiful!

A few common Item pics, and a few less common...

Phantom Dancer is joy for Tristana, high ASPD, Move Speed, and of course 1:3 chance to crit!

Ionic Spark... This is just SICK here, since with an ASPD this high, this triggers a LOT!

The Black Cleaver... Little used, but boy does it help here, AD, ASPD, and Armor ripping? (and those stack fast with that ASPD!

Zeke's Herald, Cheap life steal, ASPD, and CDR!

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Typically, I get Boots of speed to start (since mobility is powerful in the early game)
First time back, Berserker's Greaves of course, and A dagger if possible (since it builds in nicely)
I try to get Ionic Spark early, as it is VERY good for clearing out minion waves (and getting some quick Jungle Gold)
Zeal is a good start towards Phantom Dancer later, but Zeke's Herald is the next end purchase, as Lifesteal is starting to get important (and giving that to others in a team fight is just bonus candy!)
4th in line is finishing a Phantom Dancer, and reaping those benefits.
If you haven't won by now, start building that Cleaver (as they likely have gotten some good armor, and this shreds it quick)
If you really have the time and gold to build a 2nd Phantom Dancer, be ready to start racking up kills like no ones business, since you have near maxed ASPD, toss out a VERY chainable (and far reaching) Spark every second or two (which should NOT be ignored, as it easily gets kills on people who are running and think they are safe) shred armor just just by blinking at people, and crit more often than not.

Many items were kept out of this build due to just how costly they are to build, and they just don't really stand out, cost effectively.

Prmomote and Flash are the order of the day...

Promote is VERY heavily underused. It won't get you kills, but it DOES provide a powerful boost, as well as added farming. Add to that, it nearly doubles how long your minion can survive tower fire... Meaning that much longer you can spend taking down that tower, and a LOT more damage minions can do to it. VS champions, it turns a boring extra minion into an actual threat! (They didn't nerf that spell for no reason, its POWERFUL!)

Just throwing this out for people to look at and think about.

I tend to run this when I play Tristana, and its worked pretty well for me.

I'll edit/add more as I get the time :P